Interpretation of quality-of-life outcomes - Issues that affect magnitude and meaning

When quality-of-life outcomes are used to evaluate treatment effectiveness, the importance of the treatment effect relative to other clinical factors is often difficult to assess. A major methodological issue addressed in thi s review is the interpretation of quality-of-life treatment effects. The pr oblem is challenging for a number of reasons, including the subjective natu re of the quality-of-life construct, the indirect way which it is assessed, the multiple sources of measurement error, the heterogeneity of the stocha stic properties of longitudinal changes over the full range of the scale, t he complex associations among multiple outcomes, and the lack of clearly di rected therapeutic goals defined in terms of quality-of-life changes. The i nterpretation question can be addressed at 2 levels: measurement and infere nce. At the first level of measurement, it is necessary to establish the re levance of the quality-of-life metric across the distribution of changes by establishing meaningful category intervals that are important to the indiv idual patient. The second level of inference involves an evaluation of the relative benefit of a quality-of-life improvement or the risk of a quality- of-life worsening for alternative treatments in populations in whom other i ssues, such as overall cost and available health resources, must also be co nsidered. This report focuses on the quantitative issues that must be addre ssed in an interpretation of the treatment-related changes in quality-of-li fe outcomes. The conceptual framework of the problem is outlined, and probl ems that contribute to the interpretation dilemma are discussed.